Chronic pain is a major contributor to disability and is the cause of much suffering. The successful treatment of severe and chronic pain is a primary goal of the physician, with opioid analgesics being preferred drugs for doing so.
Three major classes of opioid receptors in the central nervous system (CNS) have long been known, with each class having subtype receptors. These receptor classes are known as μ, κ and δ. As opiates have a high affinity for these receptors while not being endogenous to the body, research followed in order to identify and isolate the endogenous ligands to these receptors. These ligands were identified as endorphins, dynorphins and enkephalins, respectively.
Experimentation eventually led to the identification of a cDNA encoding an opioid receptor-like (ORL-1) receptor with a high degree of homology to the known receptor classes. The ORL-1 receptor was classified as an opioid receptor based only on structural grounds, as the receptor did not exhibit pharmacological homology. It was initially demonstrated that non-selective ligands having a high affinity for μ, κ and δ receptors had low affinity for the ORL-1 receptor. This characteristic, along with the fact that an endogenous ligand had not yet been discovered, led to the term “orphan receptor.” See, e.g., Henderson et al., “The orphan opioid receptor and its endogenous ligand-nociceptin/orphanin FQ,” Trends Pharmacol. Set. 18(8):293-300 (1997).
Subsequent research led to the isolation and structure of the endogenous ligand of the ORL-1 receptor (i.e., nociceptin; also known as orphanin FQ (OFQ)). This ligand is a seventeen amino acid peptide structurally similar to members of the opioid peptide family.
The discovery of the ORL-1 receptor presents an opportunity in drug discovery for novel compounds that can be administered for pain management or other syndromes modulated by this receptor.
International PCT Publication Nos. WO 99/46260, WO 99/50254, WO 01/90102, WO 2005/028451, WO 2003/062234, and U.S. Pat. App. No. 2005/0256000, respectively, describe quinoxalines or derivatives thereof as (i) inhibitors of protein kinase C, (ii) serine protease inhibitors, (iii) herbicides, (iv) M2 acetylcholine receptor agonists, (v) medicaments for diseases involving poly(ADP-ribose) polymerase, and (vi) safeners for plants.
Citation of any reference in Section 2 of this application is not to be construed as an admission that such reference is prior art to the present application.